In a heavy duty tire, a tread portion commonly has a so-called “cap/base structure” comprising a cap rubber and a base rubber and uses a cap rubber being excellent in a wear resistance and a base rubber being excellent in an appearance and a low heat buildup during a final stage of running, in order to simultaneously establish the wear resistance and the appearance and the low heat buildup during the final stage of the running.
Regarding the base rubber, a rubber having a large breaking elongation and a low tensile stress (elastic modulus) is preferable in order to improve the appearance during the final stage of the running, and a rubber having a high tensile stress and capable of suppressing deformation during rotating is preferable in order to lower a heat buildup. Therefore, it is usually difficult to simultaneously establish the appearance and the low heat buildup of the base rubber during the final stage of the running.
As another method for lowering the heat buildup is mentioned a method of making a hysteresis loss of the base rubber low. For example, by increasing an amount of sulfur compounded in a rubber composition used in the base rubber, the hysteresis loss of the base rubber can be lowered, but the tensile stress becomes excessively large and thereby the appearance during the final stage of the running is deteriorated.
Also, by reducing an amount of a filler compounded in the rubber composition used in the base rubber, the hysteresis loss of the base rubber can be lowered and in this case, the breaking elongation of the base rubber can be also improved. However, when the amount of the filler compounded is excessively reduced, it is known that the base rubber is easily subjected to a cut from a road surface to cause deterioration of the appearance during the final stage of the running.